1. Field of the Invention
The present invention relates to a dental handpiece including a drive shaft rotatably supported interiorly thereof, which is constructed hollow at least at its operating end and is provided with an internal thread, and wherein there is located in the thus formed hollow space a tubular friction collet which is provided with an external thread and evidencing in the inoperative condition thereof inwardly bent resilient tongues formed by a plurality of slits for the receipt and the gripping of the shaft of an implement which is detachably insertable from externally thereof.
The implement can be formed by a drill, grinding or polishing tool, or the like.
2. Discussion of the Prior Art
A handpiece of that type has become known from German Patent No. 10 84 869. In this known handpiece, the external thread of the friction collet is arranged on the collet and proximate the implement and, in a similar manner, the internal thread of the hollow drive shaft is arranged on the shaft end proximate the implement whereby the tongues which are formed by the axially parallel extending slits extending to the end of the collet are located on the end of the collet remote from the implement. In order that at the insertion of the collet into the hollow drive shaft from the end of the hollow drive shaft proximate the implement, the unthreaded portion of the collet can slide unhindered past the internal thread of the drive shaft bounds are set on the outer diameter of the unthreaded collet portion so that an annular gap is formed between the last mentioned and the inner wall of the hollow drive shaft. This annular gap is thus elatively narrow, however, the thereby produced small tolerance is adequate, particularly at radially acting implement loads, to offset a tipping of the implement about an imaginary axis which extends radially in the region of the engagement of the threads which, during operation with the handpiece, will lead to an oscillating and knocking of the implement shaft located within the hollow drive shaft and, as a result, to the deformation of the shaft or the wall structure of the hollow space of the drive shaft. This will extensively and adversely influence any dental treatment. Through the continual knocking of the implement shaft, this can finally lead to the destruction of the drive shaft. In addition thereto, the clamping force of the resilient tongues of this known friction collet, in any event in the built-in condition, cannot be adjusted.
The mentioned knocking or hammering can be reduced in a handpiece as has become known from German Published Patent Application No. 19 27 743, since the collet inserted without threading into the hollow drive shaft evidences end regions which are unslitted and cylindrically-shaped, and which is retained in the collet by a special guide sleeve inserted with a press fit at each end of the collet. The inwardly bent gripping tongues are arranged in the region intermediate the cylindrically constructed end regions. The guide sleeves, as special components, represent an energy absorbent weight increase; moreover, in case of need the guide sleeves render more difficult a rapid exchange of the collet since it can mostly be disassembled and reassembled in operating with only the aid of specialized disengaging and clamping arrangements.
From German Published Patent Application No. 12 68 311 there has become known a dental handpiece which is similar to the above-mentioned handpiece, however, with the distinction that no friction collet but a threaded collet is screwed into the hollow space of the drive shaft, in essence a collet whose tongues are not inwardly bent in the inoperative condition, in effect, not prestressed. The external thread of this threaded collet is located on the end of the last-mentioned collet remote from the implement whereby the collet evidences an external conical surface cooperating with an internal conical surface of the hollow drive shaft.
Through the mutual screwing together of the collet and the drive shaft under the utilization of a key, the collet and the drive shaft are longitudinally displaced relative to each whereby, through the interaction of the internal conical surface and the external conical surface, the tongues of the collet come into gripping contact with the implement shaft and restrain the last-mentioned within the hollow drive shaft.
A handpiece with a friction collet which has become known from the above-mentioned German Patent No. 10 84 869 has the advantage that the implement shaft, upon occasion with the assistance of special pressure or thrust tools, can be withdrawn relatively rapidly from the collet grip and inserted therein. The handpiece with the threaded collet which is known from the above-mentioned German Published Patent Application No. 12 68 311 affords the advantage that through a suitably strong tightening of the threads there can be achieved an extremely rigid seating of the implement shaft.
There is thus present the requirement for the dentist that, in accordance with the kind of treatment which is to be effected, there be employed either a friction collet or a threaded collet. Heretofore, two different handpieces were required for this purpose.